Seals are not hermetic, but for most purposes adequate especially on road bikes or cleanly ridden ATB.
The seals 'loosen' over time, the grease evaporates (volatiles) or polymerizes and gradually loses effectiveness. I had a pair of Phil BB that were put in service in the mid '70s and stored from '77 to '82 and then from '84 to '99. The grease had converted to paraffin. I popped the seals out, rinsed and relubed then reinstalled the seals and the BB lasted several thousand miles til the bike was off loaded to a brother. Plastic seals are removable, metal seals are 'not' removeable without damage. A Chinese machine tool bought in '94 and sporadically used til '02 was found to have a frozen spindle. Dissassembly again showed paraffinized lube. Clean and relube solved the problem. Sealed bearings of the type in headsets would be more difficult to get at for clean/lube than BB bearings and are not designed for user maintenance, though this is in theory possible. They are a lot better protected than loose/caged ball bearing headsets from the environment.
Steve